An awn is a hairy or bristle-like growth on a plant. On the seeds of grasses such as

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

or rye, they form foxtails which assist

seed dispersal In spermatophyte plants, seed dispersal is the movement, spread or transport of seeds away from the parent plant. Plants have limited mobility and rely upon a variety of dispersal vectors to transport their seeds, including both abiotic vectors, ...

by being barbed and so sticking to passing animals. Also, the awns may twist or curl as they are wetted and dry out and this action can make fallen seeds walk until they fall into a crevice into which they then burrow. Besides grasses, other families of plants which have awns include

Asteraceae Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...

such as sunflowers and Geraniaceae such as geraniums. In the latter, the awns help disperse the seeds by developing a tension which then catapults the seeds when the seed head ripens and dries out.

Description

In grasses, awns typically extend from the lemmas of the

floret This glossary of botanical terms is a list of definitions of terms and concepts relevant to botany and plants in general. Terms of plant morphology are included here as well as at the more specific Glossary of plant morphology and Glossary ...

s. This often makes the hairy appearance of the grass synflorescence. Awns may be long (several centimeters) or short, straight or curved, single or multiple per floret. Some biological

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

are named after their awns, such as the three-awns (''Aristida''). In some

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, the awns can contribute significantly to

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

, as, for example, in

. The awns of wild emmer-wheat

spikelet A spikelet, in botany, describes the typical arrangement of the inflorescences of grasses, sedges and some other monocots. Each spikelet has one or more florets. The spikelets are further grouped into panicles or spikes. The part of the sp ...

s effectively self-cultivate by propelling themselves mechanically into

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

s. During a period of increased

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

during the night, the awns of the spikelet become erect and draw together, and in the process push the grain into the soil. During the daytime the humidity drops and the awns slacken back again; however, fine

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

hairs on the awns act as ratchet hooks in the soil and prevent the spikelets from reversing back out again. During the course of alternating stages of daytime and nighttime humidity, the awns' pumping movements, which resemble swimming frog kick, drill the spikelet as much as an inch into the soil. When awns occur in the Geraniaceae, they form the distal (rostral) points of the five

carpel Gynoecium (; ; : gynoecia) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more ...

s, lying parallel in the

style Style, or styles may refer to: Film and television * ''Style'' (2001 film), a Hindi film starring Sharman Joshi, Riya Sen, Sahil Khan and Shilpi Mudgal * ''Style'' (2002 film), a Tamil drama film * ''Style'' (2004 film), a Burmese film * '' ...

above the ovary. Depending on the species, such awns have various seed-dispersal functions, either dispersing the seed by flinging it out (seed ejection); flinging away the entire carpel so that it snaps off (carpel projection); entangling the awn or bristles on passing animals ( zoochory); or possibly burying the seed by twisting as it lies on soft soil.

References

External links

{{DEFAULTSORT:Awn (Botany) Plant morphology